I bought my turbo quite cheap on fleabay because she had exactly these symptoms.
I however knew it wasn't turbo because she had had a new Volvo original equipment turbo fitted by a volvo dealer (=megabucks) about 3,000 miles ago.
I thought it wasn't valve seals because there was no big puff on startup, nor head gasket because there was no water use nor emulsified oil.
I got a bloke to clean out the entire oil breather system, flush engine with flushing oil and refill with right oil.
Problem gone. Now doing about 1,000 to 1,500 mpp which I am told is normal.
All I need to do now is the leather, the shocks (including nivos), the resonance at 3,100 rpm in top (AW22 box) and the kickdown. Oh, and the diff grumbles slightly too. Already done sundry small paint and dent probs. Fleabay seller said she was "in excellent condition" apart from the fuming. A bit vexed with him.